Infrared (IR) Spectroscopy
• IR deals with the interaction of infrared radiation with
matter. The IR spectrum of a compound can provide
important information about its chemical nature and
• Most commonly, the spectrum is obtained by
measuring the absorption of IR radiation, although
infrared emission and reflection are also used.
• Widely applied in the analysis of organic materials, also
useful for polyatomic inorganic molecules and for
1. Electromagnetic radiation
3. Principle of IR experiment
4. IR spectrum
5. Types of vibration
6. CGF/Fingerprint regions
7. IR activity of vibrations
8. Interpretation of IR spectra
10. Sample preparation
The propagation of electromagnetic
radiation in a vacuum is constant for all
regions of the spectrum (= velocity of light):
c = λ × ν
1 Å = 10 –10 m, 1 nm = 10 –9 m, 1 μm = 10 –6 m
Another unit commonly used is the wavenumber, which is linear with energy:
Work by Einstein, Planck and Bohr indicated that electromagnetic radiation can be
regarded as a stream of particles or quanta, for which the energy is given by the
LIMIT OF RED LIGHT: 800 nm, 0.8 μm, 12500 cm-1
NEAR INFRARED: 0.8 -2.5 μm, 12500 - 4000 cm-1
MID INFRARED: 2.5 - 50 μm, 4000 - 200 cm-1
FAR INFRARED: 50 - 1000 μm, 200 - 10 cm-1
Divisions arise because of different optical materials and
Note: Horizontal scale changes at 2000 cm-1, which the unit at higher
wavenumbers being represented by half of the linear distance of those at
lower wavenumbers. The expanded region below 2000 cm-1 permits easier
identification of spectral features. Numerous IR bands usually appear in
Wavenumber scale is preferred in IR spectroscopy because of direct
proportionality between this quantity and both energy & frequency.
F= 1.2 x 1014 Hz F= 2 x 1013 Hz
Molecular spectra 8
There are three basic types of optical
spectra that we can observe for
1.Electronic spectra (Uv-visible-near IR)
(transitions between a specific vibrational and rotational level
of one electronic state and a vibrational and rotational level of
another electronic state)
2.Vibrational or vibrational-rotational
spectra (IR region)
(transitions from the rotational levels of one vibrational level
to the rotational levels of another vibrational level in the same
3.Rotational spectra (microwave region)
(transitions between rotational levels of the same vibrational
level of the same electronic state)
Change of dipole moment during vibrations & rotations
The energy associated with IR radiation is not enough for electronic transitions
like Uv-Vis radiation. Absorption of IR radiation is thus confined largely to
molecular species that have small energy differences between various vibrational
& rotational states.
To absorb IR radiation, a molecule must undergo a net change in dipole moment
as it vibrates or rotates. Only under these circumstances, the radiations interact
with the molecule and cause changes in the amplitude of its motion. E.g. the
charge distribution around a molecule such as hydrogen chloride (HCl) is not
symmetric because chlorine has higher electron density than hydrogen. Thus HCl
has significant dipole moment and is said to be polar.
As HCl molecule vibrates, a regular fluctuations in its dipole moment occurs, and a
field is established that can interact with the electric field associated with the
radiation. If the frequency of radiation exactly matches with the natural vibrational
frequency of the molecule, absorption of the radiation takes place that produces the
change in the amplitude of the molecular vibration.
No net change in dipole moment occurs
during the vibration or rotation of
homonuclear species such as O2, N2 or Cl2.
As a result, such compounds can not absorb
IR radiations. With the exception of few
compounds of this type, all other molecular
species absorb IR radiation.
• Infrared radiation in the range from 10,000 – 100 cm –1 is
absorbed and converted by an organic molecule into energy of
–> this absorption is quantized:
Vibrational spectra (I): Harmonic oscillator model
The negative sign indicates that F is a restoring force. This means
that the direction of the force is opposite to the direction of the
A simple harmonic oscillator is a mechanical system consisting of
a point mass connected to a massless spring. The mass is under
action of a restoring force proportional to the displacement of
particle from its equilibrium position and the force constant f (also
k in followings) of the spring. 13
The vibrational frequency increases with:
• increase in force constant f , increases bond strength
• decreasing atomic mass
• Example: f c≡c > f c=c > f c-
• The vibrational energy V(r) can be calculated using the (classical) model of the
• Using this potential energy function in the Schrödinger equation, the vibrational
frequency can be calculated:
If we express the radiations in wavenumber,
IR measurements in conjunction with above equations permits the
evaluation of the force constants for various types of chemical bonds.
Generally k has been found to lie in the range 3 x 102 to 8 x 102 N/m
for most single bonds, with 5 x 102 N/m serving as a reasonable
Double and triple bonds are found by this same means to have force
constant of about two to three times this value. i.e. 1 x 103 and 1.5 x 103
With these average experimental values, one can estimate the
wavenumber of the fundamental absorption band or the absorption due
to the transition from the ground state to the first excited states for a
variety of bond types.
Calculate the approximate wavenumber and the wavelength of the
fundamental absorption due to the stretching vibration of a carbonyl
The mass of the carbon atom in Kg is given by,
Similarly, for oxygen,
Vibrational spectra (II): Anharmonic oscillator model
The actual potential energy
of vibrations fits the
parabolic function fairly
well only near the
distance. The Morse
potential function more
closely resembles the
potential energy of
vibrations in a molecule for
all internuclear distances-
• The energy difference between
the transition from n to n+1
corresponds to the energy of the
absorbed light quantum
• The difference between two
adjacent energy levels gets
smaller with increasing n until
dissociation of the molecule
occurs (Dissociation energy ED )
ΔEVIB = ( En+1 – En ) =h⋅
Weaker transitions called “overtones” are sometimes observed. These correspond to
Δυ=2 or 3, and their frequencies are less than two or three times the fundamental
frequency (Δυ=1) because of anharmonicity.
Typical energy spacings for vibrational levels are on the order of 10-20 J. From the
Bolzmann distribution, it can be shown that at room temperature typically 1% or
less of the molecules are in excited states in the absence of external radiation. Thus
most absorption transitions observed at room temperature are from the υ=0 to the
υ=1 level. 20
The vibrational spectra appear as bands rather than lines. When vibrational
spectra of gaseous diatomic molecules are observed under high-resolution
conditions, each band can be found to contain a large number of closely
spaced components— band spectra. The structure observed is due to that
a single vibrational energy change is accompanied by a number of
rotational energy changes. The form of such a vibration-rotation spectrum
can be predicted from the energy levels of a vibrating-rotating molecule.
–> “vibrational-rotational bands”
Vibrational spectra (III): Rotation-vibration transitions
A vibrational absorption transition from υ to υ+1 gives rise to three sets
of lines called branches:
Lower-frequency P branch: Δυ=1, ΔJ=-1;
Higher-frequency R branch: Δυ=1, ΔJ=+1;
Q branch: branch: Δυ=1, ΔJ=0. 22
Spectrum of the Rotating Oscillator
• The selection rules allow only transitions with Δν = +1 and ΔJ = ±1
(the transition with ΔJ = 0 is normally not allowed except those with
an odd number of electrons (e.g. NO)).
The IR absorption spectrum can be obtained with gas-phase or
with condensed-phase molecules. For gas-phase molecules
vibration-rotation spectra are observed, while in condensed
phases, the rotational structure is lost.
For most routine analytical applications of infrared
spectrometry, spectra are obtained with condensed-phase
samples. Hence, we discuss here centers around the vibrational
transitions observed with molecules present as pure liquid, as
solutions, or in the solid state.
• How many vibrations are possible (=fundamental vibrations)?
A molecule has as many degrees of freedom as the total degree of
freedom of its individual atoms. Each atom has three degrees of freedom
(corresponding to the Cartesian coordinates), thus in an N-atom
molecule there will be 3N degree of freedom.
In molecules, movements of the atoms are constrained by interactions
through chemical bonds.
Translation - the movement of the entire molecule while the positions
of the atoms relative to each other remain fixed: 3 degrees of
Rotational transitions – interatomic distances remain constant but the
entire molecule rotates with respect to three mutually perpendicular
axes: 3 rotational freedom (nonlinear), 2 rotational freedom (linear).
Principle of IR experiments
•E-vector in electromagnetic radiation has frequency ν
•Molecular vibrations involving change in dipole moment set up
fluctuating electric field
Vibrational energies: fundamental (= one quantum)
•Energy transferred to molecule by resonance when vibration
frequency is the same as that of the electromagnetic radiation
• Vibrations which do not change the dipole moment are Infrared Inactive
The energy associated with a quantum of light may be transferred to the
molecule if work can be performed on the molecule in the form of
displacement of charge.
A molecule will absorb infrared radiation if the change in vibrational
states is associated with a change in the dipole moment (μ) of the
µ = qr
q: electrical charge, r: directed distance of that charge from some
defined origin of coordinates from the molecule.
Dipole moment is greater when electronegativity difference between the
atoms in a bond is greater. Some electronegativity values are:
H 2.2; C 2.55; N 3.04; O 3.44; F 3.98; P 2.19; S 2.58; Cl 3.16
• The theoretical number of fundamental vibrations (absorption
frequencies) will seldom (hardly/rarely) be observed
–> overtones (multiples of a given frequency), combination (sum of
two other vibrations) or difference (the difference of two other
vibrations) tones increase the number of bands
–> the following effects will reduce the number of theoretical
• frequencies which fall outside the measured spectral region (400-
4000 cm –1 )
• bands which are too weak
• bands are too close and coalesce / overlapping
• occurrence of a degenerate band from several absorptions of the
• lack of change in molecular dipole
Why not 3N-6/3N-5 bands in IR spectrum?
With certain functional or structural groups, it has been found
that their vibrational frequencies are nearly independent of the
rest of the molecule – group frequencies.
Carbonyl group 1650 to 1740 cm-1 various aldehydes and ketones
For many groups involving only two atoms, the approximate
frequency of the fundamental vibration can be calculated from
a simple harmonic oscillator model.
Calculations show that for most groups of interest, characteristic
frequencies of stretching vibrations should lie in the region 4000 to
1000 cm-1. In practical, the region from 4000 to 1300 cm-1 is often
called the group frequency region.
The presence of various group vibrations in the IR spectrum is of
great assistance in identifying the absorbing molecule.
In the region from ≈ 1300 to 400 cm-1, vibrational frequencies are
affected by the entire molecule, as the broader ranges for group
absorptions in the figure below – fingerprint region.
Absorption in this fingerprint region is characteristic of the molecule
as a whole. This region finds widespread use for identification purpose
by comparison with library spectra.
• When two bond oscillators share a common atom, they seldom behave
as individual oscillators (unless the individual oscillation frequencies are
The frequency of the asymmetric stretching vibration in CO2 is at a
shorter wavelength (higher frequency) than for a carbonyl group in
aliphatic ketones (around 1715 cm–1 ).
–> there must be strong mechanical coupling or interaction!
Example: C–O stretching band in
Methanol: 1034 cm –1
Ethanol: 1053 cm –1
not an isolated stretching vibration, but rather a coupled symmetric
stretching invloving C–C–O stretching
• The vibrations must be of the same symmetry
• The interaction is greatest, when the coupled groups absorb
(individually) near the same frequency --- the same energies of isolated
• Strong coupling between stretching vibrations requires a common atom
between the two groups
• Coupling between bending and stretching vibrations can occur if the
stretching bond forms one side of the changing angle.
• A common bond is required for coupling of bending vibrations.
• Coupling is negligible when groups are separated by one or more carbon
atoms and the vibrations are mutually perpendicular.
Requirements for Coupled Interactions
“A hydrogen bond exists when a hydrogen atom is bonded to two
or more other atoms”
–> not an ordinary covalent bond, since the hydrogen atom
has only one orbital (1s) to engage in covalent bonding
Typical H-bond: Hydrogen is attached to two very
electronegative atoms, usually in a linear fashion and not
=> The s orbital of the proton can effectively overlap with
the p or π orbital of the acceptor group.
• Hydrogen bonding alters the force constant of both groups:
– the X–H stretching bands move to lower frequency
– the stretching frequency of the acceptor group (B) is also reduced,
but to a lesser degree
– The X–H bending vibration usually shifts to a shorter wavelength
Effect of Hydrogen Bonding
y axis is %T or A
x axis is wavenumber (or wavelength)
Io → sample → I
T = I/Io %T = 100 I/Io
T transmission / transmittance
A = -log T
A absorbance (no units)
(Note A (but not T) ∝ concentration)
• Dispersive instruments: with a monochromator to be used in
the mid-IR region for spectral scanning and quantitative
• Fourier transform IR (FTIR) systems: widely applied and
quite popular in the far-IR and mid-IR spectrometry.
• Nondispersive instruments: use filters for wavelength
selection or an infrared-absorbing gas in the detection system
for the analysis of gas at specific wavelength.
Dispersive IR spectrophotometers
Simplified diagram of a double beam infrared spectrometer
Modern dispersive IR spectrophotometers are invariably double-beam
instruments, but many allow single-beam operation via a front-panel
Double-beam operation compensates for atmospheric absorption, for the
wavelength dependence of the source spectra radiance, the optical
efficiency of the mirrors and grating, and the detector instability, which
are serious in the IR region.⇒single-beam instruments not practical.
Double-beam operation allows a stable 100% T baseline in the spectra.
• Reflection gratings ( made from various plastics): the groove
spacing is greater (e.g. 120 grooves mm-1). To reduce the effect of
overlapping orders and stray radiation, filters or a preceding prism
are usually employed. Two or more gratings are often used with
several filters to scan a wide region.
• Mirrors but not lenses are used to focus and collimate the IR
radiation. Generally made from Pyrex or another material with low
coefficient of thermal expansion. Front surfaces coated with a
vacuum-deposited thin metal film of Al, Ag, or Au.
• Windows are used for sample cells and to permit various
compartment to be isolated from the environment.
→ transparent to IR over the wavelength region
→ inert to the various chemicals analyzed
→ capable of being shaped, ground, and polished to the desired
The Fourier transform method provides an alternatives to the use
of monochromators based on dispersion.
In convensional dispersive spectroscopy, frequencies are
separated and only a small portion is detected at any particular
instant, while the remainder is discarded. The immediate result is
a frequency-domain spectrum.
Fourier transform infrared spectroscopy generates time-domain
spectra as the immediately available data, in which the intensity is
obtained as a function of time.
Direct observation of a time-domain spectrum is not immediately
useful because it is not possible to deduce, by inspection,
frequency-domain spectra from the corresponding time-domain
waveform (Fourier transform is thus introduced).
Fourier Transform Infrared Spectrometer (FTIR)
In one arm of the interferometer, the IR source radiation travels through the beam
splitter to the fixed mirror back to the beam splitter through the sample and to the
detector. In the other arm, the IR source radiation travels to the beam splitter to the
movable mirror, back through the beam splitter to the sample and to the detector. The
difference in pathlengths of the two beams is the retardation δ. An He-Ne laser is used
as a monochromatic reference source. The laser beam is sent through the interferometer
in the opposite direction to that of the IR beam.
Single-beam FTIR Spectrometer
If moving mirror moves 1/4 λ (1/2 λ round-trip) waves are out of phase at beam-
splitting mirror - no signal
If moving mirror moves 1/2 λ (1 λ round-trip) waves are in phase at beam-splitting
mirror – signal
Difference in pathlength called retardation δ
Plot δ vs. signal - cosine wave with frequency proportional to light
frequency but signal varies at much lower frequency
One full cycle when mirror moves distance λ/2 (round-trip = λ)
Frequency of signal:
If mirror velocity is 1.5 cm/s
Bolometer, pyroelectric, photoconducting IR detectors can "see“ changes
on 10-4 s time scale!
VMM velocity of moving mirror
• very high resolution (< 0.1 cm –1 )
Two closely spaced lines only separated if one complete "beat" is recorded. As
lines get closer together, δ must increase.
Δν(cm−1) = 1/δ
Mirror motion is 1/2 δ
Resolution governed by distance movable mirror travels
• very high sensitivity (nanogram quantity)
can be coupled with GC analysis (–> measure IR spectra in gas-phase)
• High S/N ratios - high throughput
Few optics, no slits mean high intensity of light
• Rapid (<10 s)
• Reproducible and • Inexpensive
Advantages of FTIR
Usually to improve resolution
decrease slit width but less light
makes spectrum "noisier" - signal
to noise ratio (S/N)
n # scans
S/N improves with more scans
(noise is random, signal is not!)
To improve S/N ratio
For routine instrument calibration, run the spectrum of
polystyrene film (or indene) at resolution 2 cm-1. Band
positions are available in the literature.
Higher resolution calibrations may be made from gas-
phase spectra (e.g. HCl gas).
* FTIR 64
Sample preparation techniques
The preparation of samples for infrared spectrometry is often the most
challenging task in obtaining an IR spectrum. Since almost all substances absorb
IR radiation at some wave length, and solvents must be carefully chosen for the
wavelength region and the sample of interest.
Infrared spectra may be obtained for gases, liquids or
solids (neat or in solution)
• A gas sample cell consists of a cylinder of glass or sometimes a metal.
The cell is closed at both ends with an appropriate window materials
(NaCl/KBr) and equipped with valves or stopcocks for introduction of the
• Long pathlength (•10 cm) cells – used to study dilute (few molecules)
or weakly absorbing samples.
To resolve the rotational structure of the sample, the cells must be
capable of being evacuated to measure the spectrum at reduced
• For quantitative determinations with light molecules, the cell is
sometimes pressurized in order to broaden the rotational structure and
all simpler measurement.
Herriott cell - Adjust D to change the
number of passes
Circular Multipass Cell - The
beam propagates on a star
pattern. The path length can be
adjusted by changing the
incidence angle Φ.
Multipass cells – more compact and efficient instead of long-
pathlength cells. Mirrors are used so that the beam makes
several passes through the sample before exiting the cell.
(Effective pathlength • 10 m).
• Pure or soluted in transparent solvent – not water (attacks windows)
•The sample is most often in the form of liquid films (“sandwiched”
between two NaCl plates)
• Adjustable pathlength (0.015 to 1 mm) – by Teflon spacer
* FTIR 68
Regions of transparency for common infrared solvents.
The horizontal lines indicate regions where solvent transmits at least
25% of the incident radiation in a 1-mm cell.
* FTIR 69
• Spectra of solids are obtained as alkali halide discs (KBr), mulls
(e.g. Nujol, a highly refined mixture of saturated hydrocarbons) and
films (solvent or melt casting)
Alkali halide discs:
1. A milligram or less of the fine ground sample mixed with about
100 mg of dry KBr powder in a mortar or ball mill.
2. The mixture compressed in a die to form transparent disc.
1. Grinding a few milligrams of the powdered sample with a mortar
or with pulverizing equipment. A few drops of the mineral oil
added (grinding continued to form a smooth paste).
2. The IR of the paste can be obtained as the liquid sample.
* FTIR 70
1. Fundamental chemistry
Determination of molecular structure/geometry.
e.g. Determination of bond lengths, bond angles of
2. Qualitative analysis – simple, fast, nondestructive
Monitoring trace gases: Non Dispersive IR (NDIR). Rapid,
simultaneous analysis of GC, moisture, N in
soil. Analysis of fragments left at the scene of a crime
Quantitative determination of hydrocarbons on filters, in
air, or in water
Main uses of IR spectroscopy: 71
Near-infrared and Far-infrared absorption
The techniques and applications of near-infrared (NIR) and
far-infrared (FIR) spectrometry are quite different from those
discussed above for conventional, mid-IR spectrometry.
Near-infrared: 0.8 -2.5 μm, 12500 - 4000 cm-1
Mid-infrared: 2.5 - 50 μm, 4000 - 200 cm-1
Far-infrared: 50 - 1000 μm, 200 - 10 cm-1
NIR shows some similarities to UV-visible spectrophotometry and
some to mid-IR spectrometry. Indeed the spectrophotometers
used in this region are often combined UV-visible-NIR ones.
The majority of the absorption bands observed are due to
overtones (or combination) of fundamental bands that occur in
the region 3 to 6 μm, usually hydrogen-stretching vibrations.
NIR is most widely used for quantitative organic functional-group
analysis. The NIR region has also been used for qualitative
analyses and studies of hydrogen bonding, solute-solvent
interactions, organometallic compounds, and inorganic
Almost all FIR studies are now carried out with FTIR
The far-IR region can provide unique information.
i) The fundamental vibrations of many organometallic and
inorganic molecules fall in this region due to the heavy atoms
and weak bonds in these molecules.
ii) Lattice vibrations of crystalline materials occur in this region,
iii) Electron valence/conduction band transition in
semiconductors often correspond to far-IR wavelengths.
Used for smooth surfaces
Angle of reflectance = incident of reflection
For examining smooth surfaces only of solids
or coated solids.
• Specular reflectance sampling in FTIR
represents a very important technique useful
for measurement of:
- Thin films on reflective substrates.
- Analysis of bulk materials.
- Measurement of monomolecular layers on a
• Often sample analysis with no sample
Not as popular as other reflection techniques.
the angle of incidence equals the
angle of reflection
The basics of the sampling technique:
Involve measurement of the reflected energy from a sample
surface at a given angle of incidence.
The electromagnetic reflectance dependent upon:
• The angle of incidence of the illuminating beam
• The refractive index and thickness of the sample
• Experimental conditions.
Types of specular reflectance experiments
• Reflection-Absorption of relatively thin films on reflective
substrates measured at near normal angle of incidence
• Specular Reflectance measurements of relatively thick samples
measured at near normal angle of incidence
• Angle Reflection-Absorption of ultra-thin films or monolayers
deposited on surfaces measured at high angle of incidence.
Attenuated total reflectance (ATR)
• ATR accessories are especially useful for obtaining
IR spectra of difficult samples that cannot be readily
examined by the normal transmission method.
• They are suitable for studying thick or highly
absorbing solid and liquid materials, including films,
coatings, powders, threads, adhesives, polymers, and
• ATR requires little or no sample preparation for most
samples and is one of the most versatile sampling
Theory of ATR
• ATR occurs when a beam of radiation enters from a more-
dense (with a higher refractive index) into a less-dense
medium (with a lower refractive index).
• The fraction of the incident beam reflected increases
when the angle of incidence increases. All incident
radiation is completely reflected at the interface when the
angle of incidence is greater than the critical angle (a
function of refractive index).
• The beam penetrates a very short distance beyond the
interface and into the less-dense medium before the
complete reflection occurs.
• This penetration is called the evanescent wave and
typically is at a depth of a few micrometers (μm).
• Its intensity is reduced (attenuated) by the sample in
regions of the IR spectrum where the sample absorbs.
• The sample is normally placed in close contact with a
more-dense, high-refractive-index crystal such as zinc
selenide, thallium bromide–thallium iodide, or
• The IR beam is directed onto the beveled edge of the
ATR crystal and internally reflected through the
crystal with a single or multiple reflections.
• Both the number of reflections and the penetration
depth decrease with increasing angle of incidence.
• For a given angle, the higher length-to-thickness
ratio of the ATR crystal gives higher numbers of
A variety of types of ATR accessories are available:
such as 25 to 75° , vertical variable-angle ATR, horizontal
ATR, and Spectra-Tech Cylindrical Internal Reflectance
Cell for Liquid Evaluation (CIRCLE®) cell.
The resulting ATR-IR spectrum resembles the
conventional IR spectrum, but with some differences:
- Identical absorption band positions but with different
- FTIR spectrometers permit higher-quality spectra .
penetrates sample and
is partially absorbed
Penetration of a sample is
independent of its
thickness; Interference and
scattering do not occur in a
sample; Absorbance in a
sample is independent of
Total internal reflection, TIR:
Radiation strikes an interface
with a medium of lower RI,
with an angle > θc.
J. Workman, A.W. Springsteen, “Applied Spectroscopy”, Academic
J.M. Hollas, “Modern Spectroscopy”, John Wiley&Sons, 1996.
B. Stuart, W.O. George, D.J. Ando, “Modern Infrared Spectroscopy”,
John Wiley&Sons, 1997.
N.N. Colthup, L.H. Daly, S.E. Wiberly, S.E. Wiberly, “Introduction to
Infrared and Raman Spectroscopy”, Academic Press, 1997.
B. Schrader, D. Bougeard, “Infrared and Raman Spectroscopy: Methods
and Applications”, John Wiley&Sons, 1995.
* FTIR 85